Nowadays, thin lines having a line diameter of about 20 to 50 μm and formed of gold which has a 4N purity (99.99 mass %) and which has a superior electrical conductivity are mainly used as semiconductor mounting bonding wires (hereinafter called “bonding wire”) connecting an electrode mainly formed of Al on a semiconductor device with a wiring on a semiconductor circuit wiring substrate, such as a lead frame, a substrate, or a tape. In general, the relationship between the elongation of a bonding wire and stress is often evaluated through a mechanical test called tensile test, maximum stress until the bonding wire breaks out in a measurement is called tensile strength, and a maximum elongation is called breaking elongation. Here, there is a common tendency that the larger the breaking elongation becomes, the smaller the tensile strength becomes. Normally, bonding wires are often set to have a breaking elongation of 4% or so, and it is generally thought that a strength sufficient for mounting a semiconductor device cannot be obtained if the breaking elongation is set to be greater than or equal to 7%. A relationship diagram for stress obtained through the tensile test and elongation is called tensile curve. A material elastically deforms at the beginning where stress is small, and the stress, which is at a point where the stress becomes large and the material starts permanent deformation called plastic deformation, is called yield stress.
In general, a method called ball bonding is used to connect both a bonding wire and the electrode, and a method called wedge bonding is used to connect both a bonding wire and a wiring on the semiconductor circuit wiring substrate. According to the ball bonding, the leading end of a bonding wire is subjected to arc heat input to melt the leading end thereof, the meltage is solidified using surface tension to form a ball called initial ball at the leading end, the initial ball and the electrode are heated within a range from 150 to 300° C. while ultrasonic wave is applied thereto to fix those by applying pressure, thereby bonding the bonding wire and the electrode. On the other hand, according to the wedge bonding, while a bonding wire is directly heated to a temperature from 150 to 300° C., ultrasonic wave is applied to fix the bonding wire and the wiring on the substrate by applying pressure, thereby bonding the bonding wire on the wiring on the substrate. Note that a portion having undergone the ball bonding is often called first bonding (hereinafter, “1st bonding”) part, and a portion having undergone the wedge bonding is often called second bonding (hereinafter, 2nd bonding”) part. Recently, in response to the needs for high densification of a semiconductor device, miniaturization, thinning and the like, a case where a resin-based semiconductor circuit wiring substrate (e.g., BGA: Ball Grid Array substrate) is used instead of a lead frame used widely and conventionally increases. In general, when the resin-based substrate is used, it is necessary to bond a bonding wire at a lower temperature than that of the conventional lead frame (about 250° C. for the lead frame, and about 150° C. for the resin-based substrate).
A semiconductor device is packaged through a so-called resin encapsulation process of filling a thermosetting epoxy resin after a bonding wire is bonded to an electrode or a wiring and of solidifying the resin. At this time, there is the possibility of a wire sweep failure that a bonding wire abnormally deforms because of a filling resin and adjoining bonding wires contact with each other and shorted in the worst case. The general factor of this failure is because of the lack of the mechanical strength of a bonding wire, and it is important to suppress a failure (leaning failure) that a bonded bonding wire leans in a direction of an adjoining bonding wire. Accordingly, it is generally thought that ensuring a certain strength of a bonding wire is important. In order to do so, a bonding wire is often set to have a breaking elongation of 4% or so to enhance the strength (see patent literature 1).
Further, in regard to the 2nd bonding part, failures, such as peeling and disconnection, inherent to the lack of bonding strength often occur, and in such cases, the long-term reliability of the 2nd bonding part becomes insufficient. Accordingly, bonding wires often require a scheme which ensures a sufficient long-term reliability at a 2nd bonding part.
[Patent Literature 1] Unexamined Japanese Patent Application No. 2007-27335